1. Field of the Invention
The present invention relates to a method of producing a ferritic stainless steel strip which has a small intra-face anisotropy and which excels both in Lankford value (r value) and anti-ridging characteristics.
2. Description of the Related Art
In general, a ferritic stainless steel product is produced by heating a continuously-cast slab and subjecting the heated continuously-cast slab to a series of treatments including hot rolling (rough hot rolling and finish hot rolling), annealing,cold rolling and finish annealing.
Ferritic stainless steel thus produced is generally inexpensive and excellent in resistance to stress corrosion cracking and, hence, is widely used as material in fields such as cooking utensils and automotive parts, for example. This type of steel, however, is inferior to austenitic stainless steel in regard to press formability in terms of r value and anti-ridging characteristic. In addition to the r value and anti-ridging characteristics, intra-face anisotropy of the r value (referred to also as ".DELTA.r" or merely as "intra-face anisotropy") is another important factor which rules quality of press forming, since heavy earing occurs in the press product when the .DELTA.r is large.
Thus, if both press formability and intra-face anisotropy of ferritic stainless steel could be remarkably improved, such ferritic stainless steel could be substituted for austenitic stainless steel because it could sustain severe conditions of press forming which hitherto could not be withstood by ferritic stainless steel.
Unfortunately, no presently known method simultaneously improves all of these three factors, i.e., r value, anti-ridging characteristic and intra-face anisotropy, of various compositions of ferritic stainless steel.
A method has been disclosed in Japanese Patent Laid-Open No. 53-48018 and Japanese Patent Publication No. 2-7391, in which Nb and Ti are added to steels having very small C and N contents (super-low C,N steel) to improve the r value and the anti-ridging characteristics of the steel.
Meanwhile, Japanese Patent Laid-Open No. 5-179358 discloses a method in which anti-ridging characteristics are improved by hot rolling with a large draft (rolling reduction), while Japanese Patent Laid-Open No. 3-219013 discloses a method in which hot rolling with a large reduction ratio is employed to improve the r value. These methods featuring merely a large reduction ratio during hot rolling disadvantageously impair the surface of the steel sheet by creating hot-roll flaws attributable to seizure between the steel sheet and roll due to the large shearing stress that is created in the surface region of the steel strip because of the large reduction ratio.
Japanese Patent Laid-Open No. 62-10217 discloses a method in which the value of the ratio (strain rate)/(friction coefficient) is controlled to 500 or greater so as to improve anti-ridging characteristics during press forming. This method, however, fails to improve intra-face anisotropy although it can appreciably improve the anti-ridging characteristic. Furthermore, this method essentially applies a large strain rate at the low temperature region of 780.degree. to 940.degree. C., thus creating problems such as failure to catch slabs in the roll nip or inferior sheet profiles.
Thus, known methods can improve either r value or anti-ridging characteristics but cannot simultaneously improve all three factors: namely, r value, anti-ridging characteristic and intra-face anisotropy. Moreover, these known methods or proposals tend to create problems such as impairment of the surface nature, sheet catching failure and inferior sheet profile.
Japanese Patent Laid-Open No. 52-39599 teaches a method for reducing intra-face anisotropy. The improvement in intra-face anisotropy can only be achieved by strictly controlling the ratio of draft between primary cold rolling and secondary cold rolling. In particular, small values of intra-face anisotropy (.DELTA.r) such as 0.11 and 0.13 for low-C, -N steel containing Ti can be obtained only by conducting primary cold rolling at the severely high reduction ratio of 87% (reduction ratio of secondary cold rolling is 0%). Other steel compositions and other rolling conditions cannot provide intra-face anisotropy below 0.45. Furthermore, an 87% cold rolling reduction ratio is extremely high when compared with ordinary cold rolling processes and, hence, is very difficult to effect. In addition, such an extremely large reduction ratio tends to reduce dimensional precision and degrade steel sheet profile. This published specification also fails to mention anti-ridging characteristics at all. Considering that ridging is caused by a {001} hot-rolling aggregate structure generated in the core of the sheet, it is very difficult to appreciably improve the anti-ridging characteristic of the steel because the {001} hot-rolling aggregate structure will not be broken even under a severe cold-rolling reduction ratio of 87%.
Japanese Patent Laid-Open No. 54-56017 discloses that intra-face anisotropy of Al-rich ferritic stainless steel can be reduced to small values such as 0.14 or 0.21 by controlling the N content to range between 0.025% and 0.12% and by meeting the condition of 0.015&lt;N-(14/27) Al&lt;0.55%.
Thus, improving intra-face anisotropy according to known methods requires strict compositional control or severely high cold rolling reductions. Moreover, these known methods improve intra-face anisotropy without making a simultaneous improvement in r value of the steel or its anti-ridging characteristics.